2.6 Study organisms
similar morphologies and life histories. Appendicularians are singular freely swimming
millimetre-sized organisms with a direct sexual life history. Their long undulating tail
takes up most of their body, thus the alternative name larvaceans due to the morphological
similarity to larval stages of many marine organisms (gure 2.9 A). The
appendicularian body is surrounded by a mucous lter house that is used for the capture
of prey and often has a very complex structure with several chambers and dierent
kinds of lters Fenaux, 1998. Thaliaceans are colonial organisms which alternate between
sexually reproducing (blastozoid) and asexually reproducing (oozoid) life stages.
Morphological characteristics include a transparent body covered by a thin tunic, of
size ranging from a few millimetres to several centimetres. Their body is more or
less hollow and barrel-shaped with one opening each at the two opposite ends. For
prey capture a feeding ow is produced through the cavity by pumping water from
the front to the end opening such that it passes a ne-webbed mucous lter. In pyrosomes
and doliolids the feeding ow is produced through gill slits that are lined by
cilia, while salps use muscle rings to contract and pump water through a large lter
sac that stretches across the entire body chamber (gure 2.9 B). The blastozoid form
of salps usually arranges into colonies that can be up to several metres long, while the
oozoid form is freely swimming Godeaux et al., 1998. The ow produced by salps is
not only used for feeding with a low lter Reynolds number, but is also eective for
jet propulsion at a much higher swimming Reynolds number Madin, 1990; Sutherland
and Madin, 2010a,b.
Copepods are one of the most important and abundant grazers (feeding on primary
producers) in the ocean. Those small crustaceans are usually a few millimetres in size
and are found in all dierent marine and other aquatic environments Mauchline, 1998.
Examples of adult copepods are shown in gure 2.10. Morphological characteristics
of copepods include the antennules which are covered with setae (hairs) for chemo- or
mechanosensing, and several pairs of feeding and swimming appendages. The group
of calanoid copepods contains some of the most important freely swimming grazers in
the ocean Mauchline, 1998; Wadhwa, 2015.
All three feeding modes, i.e., cruise, ambush and feeding-current feeding, are found
in dierent species of freely swimming copepods, resulting in dierent trade-os concerning
the predation risk by larger predators Kirboe et al., 2014; Almeda et al.,
2017. Instead of relying on direct interception, copepods with their typical antennules
usually have a sensing apparatus which makes them able to detect prey remotely and
to increase their eective encounter zone for each feeding mode Yen et al., 1992. In
addition to mechanosensing via the setae on their antennules (gure 2.10 A), copepods
can use their feeding appendages with long, wide-spread \ngers" to eciently capture
and handle prey (gure 2.10 B) Koehl and Strickler, 1981.